Stress corrosion is a phenomenon which leads to strength degradation in optical fibers under stress. Hydrogen and water present in the environment are principle agents which combine with stress to accelerate slow crack growth in vitreous materials. Hermetically coated optical fibers have evolved to prevent stress corrosion since this is particularly detrimental to the use of fibers in adverse environments. These hermetic coatings have been applied by several methods to assure a continuous complete covering of fibers to prolong their useful life. Ideally, from the manufacturinig view point, hermetic coatings should be deposited on-line as the preform is being fiberized as it is drawn from a furnace or after the fiber is drawn while it is still hot from the furnace. On-line deposition of the hermetic coating is more economical than off-line and is compatible with subsequent in-line overcoating by polymeric materials to form a buffer layer to further protect the fiber from abrasion and permit handling of the fiber for cabling etc.
One known method of forming a hermetic coating on a fiber relies on the depositon of metals from a liquid melt or by vaporization, sputtering, or plasma ion deposition techniques. Dielectric materials such as SiN, SiON, SiC, SiOC, SnO or other metal oxides also have been deposited by chemical vapor deposition, sputtering, or RF and plasma ion depostion. Good results are obtained if the coatings are maintained in a thin region, from 100-500 A thick; this thickness has been found to provide good adhesion with mechanical and thermal compatibility with the optical fiber.
Because these films or coatings are so thin, however, it is difficult to determine the quality of the coverage, detect pinholes or areas of incomplete coverage and to detect and quantify such defects, except through indirect, inferential techniques which have been unacceptable for some high reliability applications.
Methods to detect "pinholes", include Scanning Electron Microscopy (SEM), immersion in hydrofluoric acid solutions, and dynamic fatique measurements. The SEM method requires the observation of the surface in an electron mciroscope. Clearly, this is a very slow, time consuming and expensive procedure. Practically speaking, it can only be applied to examine very small lengths of fiber. The technique of immersing samples in hydrofluoric solution relies on the highly corrosive action of hydrofluoric acid on silica materials. In a given concentration of hydrofluoric solution, silica materials and fibers rapidly dissolve. If the fiber is coated with a hermetic material resistant to attack, the dissolution is significantly reduced in rate. The rate of dissolution, however, depends on the presence or absence of pinholes, or, stated more precisely, on the density and area of defects exposed to the acid solution. Unfortunately, this technique does not directly quantify or identify the location and degree of coating of non-coverage.
Dynamic fatique measurements are made by subjecting fibers to varying rates of tensile stress. Under low rates of stress, uncoated fibers will exhibit low average strengths indicative of crack formation and growths to critical intensities. At higher rates of tensile stress application, the average strengths are higher, indicating the reduction of static fatique which is a rate limiting process. In fact, at very high rates, the average strengths are indentical to the strengths measured in the absence of water. On the other hand, a truly hermetic coating without pinholes should exhibit the same fatique behavior and average strength of measurements performed in the absence of water. This technique, again, does not quantify the density and areas of non-coverage, but only suggests that coverage is not complete.
Thus there is continuing need in the state-of-the-art for a method of detecting imcomplete coverage of and pinholes in a hermetic coating on an optical fiber which lends itself to in-line fiber fabrication procedures as well as off-line applications.